Slag granulation method and apparatus

ABSTRACT

A granulator comprises a rotary atomizer on to which the molten material to be granulated is poured in a stream. The rotation of the atomizer causes the molten material to be ejected therefrom in the form of globules. No fluid jets are used to break up the molten material. The globules pass through an enclosure and partially freeze to form granules which are collected in an annular trough. A gas is injected into the trough to induce a circumferential movement of the granules within the trough towards at least one exit from the trough.

This invention relates to a method of, and apparatus for, granulating amolten material. The material may be a metal, such as iron; a metaloxide, such as titanium oxide; a non-metal, such as slag generated as aby-product of a metals production process; or a mixture thereof.

The invention is particularly applicable to the granulation of slagtapped from an iron blast furnace. The granulated slag can be used as aPortland cement substitute in the manufacture of concrete.

In our co-pending International Publication No. WO93/06250 we havedescribed a granulator comprising an enclosure, a rotary atomiserdisposed within the enclosure, and means for delivering molten materialto the atomiser so that, in use, the material is broken into globuleswithout the use of fluid jets. The globules are dispersed within theenclosure where they are at least partially frozen to form granuleswhich are collected in an annular open-topped trough surrounding theatomiser.

The path taken by the globules from the atomiser to the open-toppedtrough is necessarily rather long so that the globules have time topartially freeze and form granules and so the distance from the atomiserto the outside of the open-topped trough is correspondingly long. Thismeans that a relatively large enclosure is required to house theatomiser and the open-topped trough.

It is an object of the present invention to provide such apparatus butwhich is more compact than that described in the Internationalapplication.

According to a first aspect of the present invention a granulatorcomprises an enclosure, a rotary atomiser disposed within the enclosure,means for delivering molten material to the atomiser so that, in use,the material is broken into globules without the use of fluid jets andthe globules are dispersed within the enclosure;

means providing an annular gaseous curtain with entrained granulesaround the atomiser and through which the greater part of the globulespass and partially freeze to form granules and an open-topped trough inwhich the granules are collected and from which they are removed to theexterior of the enclosure.

According to a second aspect of the present invention in a method ofgranulating a molten material, a stream of the molten material isdelivered to a rotating atomiser disposed within an enclosure; the speedof rotation of the atomiser is such that the molten material is ejectedfrom the atomiser in the form of globules without the use of fluid jets,

an annular gaseous curtain with entrained granules is provided aroundthe atomiser such that the greater part of the quantity of globulesejected from the atomiser pass through the curtain which accelerates thecooling of the globules helping them to partially freeze to formgranules and reduces the kinetic energy of the granules, the granulesbeing collected in an open-topped trough from which they are removed tothe exterior of the enclosure.

In use, the majority of the globules dispersed within the enclosure passthrough the gaseous curtain with the entrained granules and this curtainreduces the temperature of the globules causing them to become partiallyfrozen granules and it also reduces the kinetic energy of the granulescausing the length of their flight path to be reduced. The reduction inenergy of the granules allows the annular trough to be positioned closerto the atomiser than is the case where the annular gaseous curtain withentrained granules is not provided. This has the advantage that thedimensions of the enclosure can be reduced with a subsequent reductionin cost of the granulator.

The gas which is used to form the curtain is usually air and this mayalready have been passed through the granules previously collected inthe annular trough and the already heated air is further heated by thepassage of the globules therethrough raising the temperature of the airto a temperature high enough for this air exiting from the enclosure tobe used, for example, for steam raising.

The air curtain may be formed at the side of the trough adjacent theatomiser and a proportion of the granules in the trough are caught upwith the air jets so as to be entrained therewith and these granulesalong with the partially frozen globules from the atomiser fall backinto the trough and from which they are eventually removed.

In one embodiment of the invention the partially frozen granules whichpenetrate the gaseous curtain fall on to a diverter above the annulartrough and some of the granules are entrained in the air curtain whileothers flow down a plurality of stand pipes to a fluidised bed or afluidised or other bed having circumferential movement in the troughbeneath the diverter.

In another embodiment of the invention the trough contains upper andlower beds and the partially frozen granules which penetrate through theair curtain are deposited in the upper bed from which a proportion ofthem will be entrained in the air curtain and others pass through aplurality of stand pipes to the lower bed from which they are eventuallyremoved.

Alternatively the annular gaseous curtain may be positioned outside ofthe annular trough. This means that the granules passing through thecurtain are slowed down and land outside of the curtain. The granulesthen have to return in the direction towards the atomiser and arecollected in the trough.

Thus in a further embodiment of the invention, an annular cooling bed isprovided in the enclosure outside of and at a higher level than the opentopped trough. A surface is inclined downwardly from the annular coolingbed to the trough. The granules passing through the curtain fall ontothe cooling bed and build up the depth of the bed. As the depthincreases, granules will fall down the inclined surface into the trough.The annular cooling bed is conveniently positioned adjacent the outerwall of the enclosure so that the distance between the atomiser and theouter wall is reduced thereby reducing the overall size of thegranulator.

A rotary atomiser conveniently comprises a thick disc of refractorymaterial having a shaped top surface or a cooled dish of stainless steelor other metal which promotes the desired slag globule trajectories. Thetop surface may be dished to a greater or lesser extent depending upondesired trajectory, slag flow rate, cup speed range specification anddesired granule size. The device is rotated about a vertical axis bymeans of an electric or hydraulic motor. The speed of rotation of thedevice is controlled as a function of slag flow rate, higher flowsrequiring higher rotational speed to maintain globule trajectory andsize distribution. For this reason, the motor is a variable speed motor.

The globules of molten material ejected from the rotating atomiser flyoutwardly towards the surrounding walls of the enclosure. The pathstaken by the globules depend to a certain extent on the size of theglobules. The globules spread out in the vertical plane as well as inthe horizontal plane. The globules thus become in contact with the airin the enclosure and heat transfer between the globules and the airduring the movement of the globules causes the globules to at leastpartially solidify to form granules. It is arranged that most of thegranules will pass through the gaseous curtain with the entrainedgranules in it, thus slowing down the movement of the granules from theatomiser. Most of the granules will then fall directly into either theannular trough or the annular bed although others will first impingeagainst the side wall of the enclosure before falling into the trough orbed. However, as the speed of the globules from the atomiser is reducedby impact with the entrained granules in the gaseous curtain, the sizeof the enclosure can be reduced making it more compact and, therefore,of lower cost.

It is desirable for the side wall of the enclosure to include a partwhich is at a higher level than the top of the trough or the top of theannular cooling bed, said higher part being of annular form with thelower end thereof leading to the top of the trough or bed. This part ofthe side wall is conveniently liquid cooled. Alternatively, the part ofthe side wall may have downwardly directed openings therein and meansare provided for directing cooling gas through the openings into theenclosure. The cooled wall or the air directed through the openings inthe wall serve to prevent granules sticking to the wall and the granulesleave the wall and fall into the trough.

Some of the globules/granules ejected from the atomiser will fall shortof the trough. Preferably, an inclined surface extends downwardly fromthe rotary atomiser to the adjacent top edge of the trough. This surfaceconveniently has openings it is and gas is directed through theopenings. The gas and the inclined surface encourage the granulesdeposited on the surface to move to the trough where most of them willbe caught up by the gaseous curtain.

In order that the invention may be more readily understood, it will nowbe described, by way of example only, with reference to the accompanyingdrawings, in which:

FIG. 1 is a sectional elevation through a granulator in accordance withone embodiment of the invention;

FIGS. 2, 3 and 4 are each a diagrammatic sectional elevation throughpart of the apparatus of FIG. 1 showing alternative embodiments of theinvention;

FIG. 5 is a sectional elevation of apparatus in accordance with a stillfurther embodiment of the invention and

FIG. 6 is a plan of the apparatus shown in FIG. 5 on the line 6--6 ofFIG. 5.

Referring to FIG. 1, the rotary atomiser 1 comprises a stationaryvariable speed motor having a vertical drive shaft. Mounted on the driveshaft is a flange 3 supporting an atomiser dish or cup 4. The dish orcup has an upwardly facing concave surface and it is rotated about avertical axis. The speed of rotation of the dish or cup can be varied.The atomiser is located within a generally cylindrical enclosure 6 and afeed trough or runner 7 extends from outside the enclosure to a positionabove the atomiser 1. An outlet nozzle 8 permits molten material flowingdown the trough or runner 7 to fall as a stream on to the concavesurface of the cup 4. Surrounding the atomiser 1 there is an annulartrough 10 and the top of the trough is at a lower level than the cup 4.A frusto-conical surface 11 extends downwardly from the rotary atomiserto the adjacent top edge of the trough. The part 6A of the side wall,which is at a higher level than the top of the trough, has its lower endleading to the top of the trough. The enclosure is covered by a topstructure 12 through which extends at least one discharge pipe 14. Thepart 6A of the side wall of the enclosure may have a water jacketmounted on the outside thereof with provision for cooling liquid to becirculated through the jacket in order to cool the wall. However, in thearrangement shown, this part of the side wall has downwardly directedopenings 15 therethrough and a hollow casing behind the wall enables acooling gas, usually air, to be directed through the openings into theenclosure. The flow of air is downwardly towards the trough 10.Similarly, the frusto-conical surface 11 has openings therethrough and acasing below the surface enables gas, usually air, to be directedthrough the openings to the upper side of the surface. The direction ofthe air flow through the openings is towards the trough 10. Within thetrough 10, means are provided to produce a mobile bed of granules.

In the embodiment, shown in FIG. 2, a bed of granules is supported on adistributor 16 adjacent the base of the trough 10 and air under pressureis directed to the distributor in order to cause a circumferentialmovement of the granules within the trough. At the side of the trough 10which is adjacent to the atomiser 1, means are provided for producing avertically extending substantially annular air curtain 17. This curtainentrains granules from the bed with it so that the curtain with thegranules entrained in it forms an obstacle to many of the globules whichare passed outwardly from the rotating cup 4. This curtain slows downthe globules as well as cooling them so that they are at least partiallyfrozen and of reduced kinetic energy. The granules entrained with theair curtain and the granules from the rotating cup fall back into thebed in the trough 10. Granules which land on the surface 11 slide downinto the trough.

Referring now to FIG. 3, there is a diverter 18 provided at the open topof the trough 10. The air curtain 17 is formed at the side of thediverter 18 and the granules from the atomiser are slowed in flight bycollision with the curtain of air having the solidified slag entrainedwith it and the falling granules are separated by the diverter so thatsome of them are caught up with the air curtain 17 to be entrained intothe curtain while others fall through one or more stand pipes 19 ontothe bed supported on the distributor 16.

In the arrangement shown in FIG. 4 the annular trough 10 contains alower bed on the distributor 16 and an upper bed supported on a seconddistributor 20. The air curtain 17 is formed adjacent the upper bed andgranules from this bed are entrained with the air curtain. When theheight of bed on the distributor 20 reaches a certain level, furthergranules flow down one or more stand pipes 19 to the lower bed fromwhere the granules are continuously removed.

In the embodiments of the invention shown in FIGS. 2 and 3, the bedsupported on the distributor 16 is kept at a temperature suitable fordischarge and the granules are continuously removed from this bed. Thegranules, supported on the diverter 18 in the case of the embodiment ofFIG. 3 and in the upper bed of the arrangement of FIG. 4, are kept at asignificantly higher temperature. In this way, the temperature of theair in the enclosure is such that the air, on being removed from theenclosure, can be used for steam raising or other process steps.

The beds in the trough 10 may be fluidised but, conveniently, they aremobile beds moving around the trough.

The air curtain 17 is shown as being located at the side of the troughadjacent the atomiser, but the air curtain may be located outwardly fromthis side of the trough so that only the granules which would not fallinto the trough pass through the curtain to reduce their kinetic energyand thereby cause them to fall into the trough. If the air curtain 17 islocated adjacent to the side wall 6, the holes in the wall, which are analternative to liquid cooling, are directed upwardly so that the airissuing from the holes does not oppose the motion of the curtain.

The embodiment of the invention shown in FIGS. 5 and 6 allows theenclosure to be reduced in size still further by reducing the kineticenergy of the globules/granules produced by the atomiser. As shown inFIGS. 5 and 6 the atomiser, which has a water cooled cup 4, is locatedcentrally of the enclosure and surrounding and close to the atomiser isan open-topped trough 22. Stand-pipes 23 project upwardly through thebase of the trough and the base is in sections with each sectioninclined downwardly towards the nearest stand-pipe. The lower ends ofthe stand-pipes are above conveyors 24 which extend beneath theenclosure to a common conveyor 25 which is also outside of theenclosure.

Surrounding the trough 22 and at a higher level then the trough is anannular cooling bed 26 and the inner edge of the cooling bed 26 and theouter edge of the trough are connected by an inclined surface 27. Thesurface 27 is apertured with the apertures being inclined upwardlytowards the cooling bed. The cooling bed has apertures in its base andthese apertures are inclined to the base so that air directed throughthe apertures causes granules of the material supported on the bed tomove in one circumferential direction as indicated by arrows 28. Thusthe annular cooling bed comprises a circumferentially mobile bed. Thecooling bed 26 can slope gently downwardly radially towards the atomiserso that granules tend to move inwardly towards the inclined surface 27.The bed is located adjacent a side wall of the enclosure and above thebed the wall 29 of the enclosure is inclined sharply towards theatomiser.

In use, air under pressure is directed through the apertures in theinclined surface 27 and it forms an air curtain 30 located between thetrough and the cooling bed 26. The air curtain sweeps up the inclinedsurface 27 and into the space between the cooling bed 26 and theatomiser 4. Granules which have previously been formed are caught upwith the air curtain. Globules of molten material are dispersed from theatomiser and most of the globules pass through the air curtain on theirflight path. The curtain serves to accelerate the cooling of theglobules to form granules and the existence of the granules previouslyswept up into the air curtain bring about collisions with many of thenewly formed granules thereby reducing their kinetic energy. The lengthof the flight path of these granules is reduced and most of the granulesfall onto the bed 26. Others will fall on the surface 27 and some willimpinge on the enclosure wall 29 before dropping onto the bed 26. Someof those granules which fall onto the surface 27 will move down into thetrough 22 whereas others will be swept up the surface onto the bed 26.At the same time, the height of the bed 26 will be increased as moregranules are added to it and many granules will move down the surface 27into the trough 22. The granules in the trough are moved continuously toone or other of the stand-pipe 23 through which they exit the enclosureonto the conveyors 24 and 25.

A small quantity of globules leaving the atomiser cup will fall directlyinto the trough. This is due to the globules not attaining the requiredhorizontal component of velocity as they leave the cup and isundesirable. This can largely be avoided by controlling the speed ofrotation of the atomiser cup in response to the flow rate of the moltenmaterial. An increase in flow rate brings about an increase in speed.

The hot air generated in the enclosure passes out of the enclosurethrough the or each discharge pipe. To control the solidification of theglobules in the event of a surge in flow of molten material above thedesigned flow rate, and to prevent globules sticking to the enclosurewall and to each other in the bed, at least one system comprising a pipewith a plurality of nozzles 33 (see FIG. 1) is located in the enclosure,conveniently adjacent to the cup 4. Water is circulated through thenozzles in the form of a very fine mist. The mist is evaporated in thevicinity of the atomiser by the heat present in the globules to reducethe globule temperature and consequently the air temperature. A controlcircuit may be used to vary the quantity of liquid issued as a mist inaccordance with the detected temperature within the enclosure.

If heat recovery is required, the annular depression formed by theannular trough, the surface 27, and the atomiser structure, may beallowed to fill with granules. By redistributing and reducing the air tothe enclosure, granules may be discharged from the trough at the sametemperature as before whilst the resultand air temperature may beincreased to a level suitable for heat recovery or steam raising.

I claim:
 1. A granulator comprisingan enclosure (6,29); a rotaryatomiser (1) disposed within the enclosure; means (7) for deliveringmolten material to the atomiser (1) so that, in use, the material isbroken into globules without the use of fluid jets and the globules aredispersed with the enclosure and partially frozen to form granules; anopen-topped trough (10, 22) in which the granules are collected and fromwhich they are removed to the exterior of the enclosure; and means forupwardly projecting an annular curtain (17, 30) of gas with entrainedgranules around the atomiser such that the majority of the globules passthrough the curtain to reduce their temperature and to reduce thekinetic energy of the granules so formed.
 2. A granulator as claimed inclaim 1 in which the open-topped trough is of annular form and surroundsthe rotary atomiser and said means provide the gaseous curtain betweenthe atomiser and the open-topped trough.
 3. A granulator as claimed inclaim 1 in which the open-topped trough is of annular form and surroundsthe rotary atomiser and said means provide the gaseous curtain at theside of the open-topped trough which is away from the atomiser.
 4. Agranulator as claimed in claim 2 in which a surface extends downwardlyfrom the atomiser to the trough and said means provides the gaseouscurtain at a region adjacent the junction of the surface and the trough.5. A granulator as claimed in claim 4 in which said surface has openingstherein and means for directing gas through the openings towards thelower end of the surface.
 6. A granulator as claimed in claim 4 in whichmeans are provided in the trough to form either a fluidised bed ofgranules therein or a circumferential movement of a bed of granules inthe trough.
 7. A granulator as claimed in claim 4 in which exit from thetrough is provided by a conduit extending through the base of thetrough.
 8. A granulator as claimed in claim 6 in which there is adistributor in the trough above the base of the trough and granulescollected on the distributor are transferred to the bed in the base ofthe trough.
 9. A granulator as claimed in claim 3 in which an annularcooling bed is provided in the enclosure outside of and at a higherlevel than the open-topped trough and a surface is inclined downwardlyfrom the cooling bed to the trough.
 10. A granulator as claimed in claim9 in which said downwardly inclined surface is apertured and means areprovided for passing gas through said apertures to form said curtain.11. A granulator claimed in claim 9 in which said annular cooling bed islocated within the enclosure adjacent an outer wall thereof, said outerwall being inclined inwardly towards said atomiser.
 12. A granulator asclaimed in claim 1 including means for introducing a water mist into theenclosure to remove heat therefrom by evaporation.
 13. A method ofgranulating a molten material comprising the steps ofdelivering a streamof the molten material to a rotating atomiser disposed within anenclosure; adjusting the speed rotation of the atomiser such thatwithout the use of fluid jets the molten material is ejected from theatomiser in the form of globules; providing an upwardly extendingannular curtain of gas with entrained granules of frozen globules of thematerial, said curtain surrounding the atomiser, whereby the majority ofthe globules ejected from the atomiser pass through the curtain of gasto reduce their temperature and to reduce the kinetic energy of thegranules so formed; collecting the granules in an open-topped trough andremoving the granules from the trough to the exterior of the enclosure.14. A method as claimed in claim 13 in which the annular gaseous curtainis provided between the atomiser and the open-topped trough.
 15. Amethod as claimed in claim 13 in which substantially all the globulespassing through the curtain land in an annular cooling bed arrangedoutside of and at a higher level than the open-tapped trough andgranules move from the annular cooling bed to the trough.
 16. A methodas claimed in claim 15 in which the granules of the annular cooling bedmove circumferentially on the bed and granules are continuously movingdown an inclined surface from the cooling bed to the trough.
 17. Amethod as claimed in claim 16 in which some of the granules moving fromthe annular cooling bed to the trough are entrained in the gaseouscurtain.
 18. A method as claimed in claim 13 in which the granules inthe trough are moved circumferentially towards the or each exit from thetrough.
 19. A method as claimed in claim 13 in which excessivetemperature within the enclosure is controlled by injecting water mistinto the enclosure to remove heat therefrom by evaporation of the watermist.
 20. A method as claimed in claim 13 in which gas at elevatedtemperature is withdrawn from the enclosure and heat is recovered fromthe gas.